1Each frequency band has both
advantages and disadvantages. At low frequencies the signal
propagates along the ground; attenuation is low but atmospheric
noise levels are high. Low frequencies cannot carry enough
information for video services. At higher frequencies there is less
atmospheric noise but more attenuation, and a clear line of sight
is needed between the transmitter and receiver because the signals
cannot penetrate objects. These frequencies offer greater
bandwidth, or channel capacity.
1.2.1 Communications Before the
Industrial Age
The annals of antiquity offer examples of muscle-powered
communications: human runners, homing pigeons, and horse relays.
Perhaps the earliest communications infrastructure was the road
network of Rome, which carried not only the legions needed to
enforce the emperor's will
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TABLE 1-1 Overview of Wireless Radio Frequency
Communications Systems and Services
Frequency Banda
Communications Applications
Characteristics
3–30 kHz (very low, or VLF); 30–300
kHz (low, or LF)
Long-range navigation, marine radio beacons
Low attenuation, high atmospheric noise
300–3000 kHz (medium, or MF); 3–30 MHz
(high, or HF)
Maritime radio, AM radio, telephone, telegraph,
facsimile
Attenuation varies, noise drops at 30 MHz
30–300 MHz (very high, or VHF); 0.3–3
GHz (ultrahigh, or UHF)
VHF television, FM two-way radio, UHF television,
radar
Cosmic noise, line-of-sight propagation
3–30 GHz (superhigh, or SHF)
Satellite, radar, microwave
Atmospheric attenuation
30–300 GHz (extremely high, or EHF)
Experimental satellite, radar
Line-of-sight propagation
aFrequencies are in kilohertz (kHz), megahertz (MHz),
and gigahertz (GHz).
SOURCE: Adapted from Couch (1995).
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but also messengers to direct forces far from the capital.
Ancient societies also developed systems that obviated the need for
physical delivery of information. These systems operated within
line-of-sight distances (later extended by telescope): smoke
signals, torch signaling, flashing mirrors, signal flares, and
semaphore flags (Holzman and Pehrson, 1995). Observation stations
were established along hilltops or roads to relay messages across
great distances.
1.2.2 Telegraphy
The first comprehensive infrastructure for transmitting messages
faster than the fastest form of transportation was the optical
telegraph, developed in 1793. Napoleon considered this his secret
weapon because it brought him news in Paris and allowed him to
control his armies beyond the borders of France. The optical
telegraph consisted of a set of articulated arms that encoded
hundreds of symbols in defined positions. Under a military
contract, the signaling stations were deployed on strategic
hilltops throughout France, linking Paris to its frontiers. By the
mid-1800s, 556 stations enabled transmissions across more than
5,000 kilometers (km).
The optical telegraph was superseded by the electrical telegraph
in 1838, when Samuel Morse developed his dot-and-dash code. Now
information could be transmitted beyond visible distances without
significant delay. In an 1844 demonstration on a government-funded
research testbed, Morse sent the message "What Hath God Wrought?"
from Baltimore to the U.S. Capitol (Bray, 1995).
The rapid deployment of telegraphic lines around the world was
driven by the need of nineteenth-century European powers to
communicate with their colonial possessions. High-risk technology
investments were required. After the use of rubber coating was
demonstrated on cables deployed across the Rhine River, the first
transatlantic cable was laid in 1858, but it failed within months.
A new cable designed by Lord Kelvin was laid in 1866 and operated
successfully on a continuous basis.
The result was a rapidly expanding telegraphic network that
reached every corner of the globe. By 1870, Great Britain
communicated directly with North America, Europe, the Middle East,
and India. Other nations scrambled to duplicate that system's
global reach, for no nation could trust its critical command
messages to the telegraphic lines of a foreign power.
1.2.3 Early Wireless
Within a few decades of its widespread deployment, telegraphy
began to lose customers to a new technology—radio. In 1895
Guglielmo
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Marconi demonstrated that electromagnetic radiation could be
detected at a distance. Great Britain's Royal Navy was an early and
enthusiastic customer of the company that Marconi created to
develop radio communications. In 1901 Marconi bridged the Atlantic
Ocean by radio, and regular commercial service was initiated in
1907 (Masini, 1996).
The importance of this new technology became evident with the
onset of World War I. Soon after hostilities began, the British cut
Germany's overseas telegraphic cables and destroyed its radio
stations. Then Germany cut Britain's overland cables to India and
those crossing the Baltic to Russia. Britain enlisted Marconi to
put together a string of radio stations quickly to reestablish
communications with its overseas possessions.
The original Marconi radios were soon replaced by more advanced
equipment that exploited the vacuum tube's capability to amplify
signals and operate at higher frequencies than did older systems.
In 1915 the first wireless voice transmission between New York and
San Francisco signaled the beginning of the convergence of radio
and telephony. The first commercial radio broadcast followed in
1920 (Lewis, 1993). The use of higher frequencies (called
shortwaves) exploited the ionosphere as a reflector, greatly
increasing the range of communications. By World War II, shortwave
radio had developed to the point where small radio sets could be
installed in trucks or jeeps or carried by a single soldier. The
first portable two-way radio, the Handie-Talkie, appeared in 1940.
Two-way mobile communications on a large scale revolutionized
warfare, allowing for mobile operations coordinated over large
areas.
1.2.4 Telephony
The telephone was first demonstrated in 1876. A telephone
network based on mechanical switches and copper wires then grew
rapidly. The high cost of the cables limited the number of
conversations possible at any one time; as demand increased,
multiplexing techniques, such as time division and frequency
division, were developed.
A mix of independent operators ran telephone services in the
early days. Subscribers to different services could not call each
other even when in the same town. In 1913 the U.S. government
allowed American Telephone and Telegraph (AT&T) to assume
control of the national telephone network in return for becoming a
regulated monopoly delivering "universal" service. Yet it was not
until the 1950s that unified network signaling was offered to
subscribers, allowing them to make direct-dial long-distance
telephone calls (Calhoun, 1992). Since then, the rapid extension of
the long-distance telephone network has been made possible by
advances in photonic communications and network control
technologies.
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1.2.5 Communications Satellites
The concept of using geosynchronous satellites for
communications purposes was first suggested in 1945 by the science
fiction writer Arthur C. Clarke, then employed at Britain's Royal
Aircraft Establishment, part of the Ministry of Defence. Satellites
of this type are positioned above the equator and move in synch
with Earth's rotation. In 1954 J.R. Pierce at AT&T's Bell
Telephone Laboratories developed the concept of orbital radio
relays and identified the key design issues for satellites: passive
versus active transmission, station keeping, attitude control, and
remote vehicle control (Bray, 1995). Pierce advocated an approach
of reaching geostationary orbit in successive stages of technology
development, starting with nonsynchronous, low-orbit satellites.
Hughes Aircraft Company advocated a geostationary concept based on
the company's patented station-keeping techniques.
In 1957 the Soviet Union launched Sputnik, the first satellite
to be placed in orbit. Amateur radio operators were able to pick up
its low-power transmissions all over the world. In 1960 the
National Aeronautics and Space Administration (NASA) and Bell
Laboratories launched the first U.S. communications satellite,
Echo-1, in a low Earth orbit. The first satellite-based voice
message was sent by President Dwight Eisenhower using passive
transmission techniques. The next advance in satellite technology
was the successful launch of the TELSTAR system by NASA and Bell
Laboratories. Using active transmission technology TELSTAR
delivered the first television transmission across the Atlantic in
1962. Because it was placed in an elliptical orbit that varied from
low to medium altitudes, the satellite was visible
contemporaneously to Earth stations on both sides of the Atlantic
for only about 30 minutes at a time. Clearly geostationary orbits
were desirable if satellites were to be used for continuous
telephone and television communications across long distances.
In 1963 Hughes Aircraft and NASA achieved geosynchronous orbit
(known as GEO today) with the successful launch of the SYNCOM
satellite. The satellite was placed in an orbit of approximately
36,210 km, a distance that allowed it to remain stationary over a
given point on Earth's surface. SYNCOM led the way for the next
several decades of satellite systems by demonstrating that
synchronous orbit was achievable, and that station keeping and
attitude control were feasible. Today most satellites, both
military and commercial, are of the GEO variety.
COMSAT was formed by an act of Congress in 1962 and represented
U.S. commercial interests in satellite technology development at
Intelsat, established in 1964 as an international,
government-chartered organization to coordinate worldwide satellite
communications issues. INTELSAT-II (Early Bird) was launched into a
geosynchronous orbit in
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1965 and supported 240 telephone links or one television
channel. Channel capacities are now measured in the tens of
thousands of voice channels (the INTELSAT-VI, launched in 1987,
supports 80,000 voice channels).
The first military satellites, the DSCS-I group, were launched
by the U.S. Air Force in 1966. Three launches placed 26 lightweight
(100-pound) satellites in near-geosynchronous orbit. These systems
supported digital voice and data communications using
spread-spectrum technology (an important signal-processing approach
discussed extensively in Chapter 2). The satellites were replaced
in the 1970s by the DSCS-II group, which increased channel capacity
by using spot-beam antennas with high gain to boost the received
power. The first cross-linked military satellites, the LES 8/9,
were launched in 1976. This demonstration fostered a vision of
space-based architectures—without vulnerable ground
relays—for communication, navigation, surveillance, and
reconnaissance.
Satellites offer several advantages over land-based
communications systems. Rapid, two-way communications can be
established over wide areas with only a single relay in space, and
global coverage with only a few relay hops. Earth stations can now
be set up and moved quickly. Furthermore, satellite systems are
virtually immune to impairments such as multipath fading (channel
impairments are discussed in Chapter 2). But with the rapid
deployment of undersea fiber-optic links, the use of satellite
channels for telephony has been on the decline. The high capacity
of fiber provides for competitive costs, which, combined with low
latency, have attracted consumers. The future of the satellite
industry depends on the emergence of applications other than fixed
telephony channels. A new generation of satellite systems is being
deployed to provide mobile telephone services (see Section
1.5).
1.2.6 Mobile Radio and the Origins of
Cellular Telephony
The early development of mobile radio was driven by public
safety needs. In 1921 Detroit became the first city to experiment
with radio-dispatched police cars. However, transmission from
vehicles was limited by the difficulty of producing small,
low-power transmitters suitable for use in automobiles. Two-way
systems were first deployed in Bayonne, New Jersey, in the 1930s.
The system operated in "push-to-talk" (i.e., half-duplex) mode;
simultaneous transmission and reception, or full-duplex mode, was
not possible at the time (Calhoun, 1988).
Frequency modulation (FM), invented in 1935, virtually
eliminated background static while reducing the need for high
transmission power, thus enabling the development of low-power
transmitters and receivers for use in vehicles. World War II
stimulated commercial FM manufacturing capacity and the rapid
development of mobile radio technology. The
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need for thousands of portable communicators accelerated
advances in system packaging and reliability and reduced costs. In
1946 public mobile telephone service was introduced in 25 cities
across the United States. The initial systems used a central
transmitter to cover a metropolitan area. The inefficient use of
spectrum and the coarseness of the electronic filters severely
limited capacity: Thirty years after the introduction of mobile
telephone service the New York system could support only 543
users.
A solution to this problem emerged in the 1970s when researchers
at Bell Laboratories developed the concept of the cellular
telephone system, in which a geographical area is divided into
adjacent, non-overlapping, hexagonal-shaped "cells." Each cell has
its own transmitter and receiver (called a base station) to
communicate with the mobile units in that cell; a mobile switching
station coordinates the handoff of mobile units crossing cell
boundaries. Throughout the geographical area, portions of the radio
spectrum are reused, greatly expanding system capacity but also
increasing infrastructure complexity and cost.
In the years following the establishment of the mobile telephone
service, AT&T submitted numerous proposals to the Federal
Communications Commission (FCC) for a dedicated block of spectrum
for mobile communications. Other than allowing experimental systems
in Chicago and Washington, D.C., the FCC made no allocations for
mobile systems until 1983, when the first commercial cellular
system—the advanced mobile phone system (AMPS)—was
established in Chicago. Cellular technology became highly
successful commercially with the miniaturization of subscriber
handsets.
1.2.7 The Internet and Packet
Radio
The original concepts underlying the Internet were developed in
the mid-1960s at what is now the Defense Advanced Research Projects
Agency (DARPA), then known as ARPA. The original application was
the ARPANET, which was established in 1969 to provide survivable
computer communications networks. The ARPANET relied heavily on
packet switching concepts developed in the 1960s at the
Massachusetts Institute of Technology, the RAND Corporation, and
Great Britain's National Physical Laboratory (Kahn et al., 1978;
Hafner and Lyon, 1996; Leiner et al., 1997). This approach was a
departure from the circuit-switching systems used in telephone
networks (see Box 1-1).
The first ARPANET node was located at the University of
California at Los Angeles. Additional nodes were soon established
at Stanford Research Institute (now SRI International), the
University of California at Santa Barbara, and the University of
Utah. The development of a host-to-host protocol,2the network control protocol (NCP),
followed in 1970,
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BOX 1-1
Circuit Switching Versus Packet Switching
Telephone systems are based on a connection-oriented or
circuit-switched model in which connections are fixed for the
duration of a call. Such systems are inefficient when transmission
occurs in short bursts separated by long pauses. Packet switching
replaces the centralized switches with distributed routers, each
with multiple connections to adjacent routers. Messages are divided
into "packets" that are independently routed on a hop-by-hop ba is.
Such an approach allows messages to be multiplexed over the
available paths on a statistically determined basis, gracefully
adapting the transmissions to traffic level, and optimizing the use
of existing link capacity without pre-allocating link
bandwidth.
enabling network users to develop applications. At the same
time, the ALOHA Project at the University of Hawaii was
investigating packet-switched networks over fixed-site radio links.
The ALOHANET began operating in 1970, providing the first
demonstration of packet radio access in a data network (Abramson,
1985). The contention protocols used in ALOHANET served as the
basis for the "carrier-sense multiple access with collision
detection" (CSMA/CD) protocols used in the Ethernet local area
network (LAN) developed at Xerox Palo Alto Research Center in 1973.
The widespread use of Ethernet LANs to connect personal computers
(PCs) and workstations allowed broad access to the Internet, a term
that emerged in the late 1970s with the design of the Internet
protocol (IP). The need to link wired, packet radio, and satellite
networks led to the specifications for the transmission control
protocol (TCP), which replaced NCP and shifted the responsibility
for transmission from the network to the end hosts, thereby
enabling the protocol to operate no matter how unreliable the
underlying links.3
The development of microprocessors, surface acoustic wave
filters, and communications protocols for intelligent management of
the shared radio channel contributed to the advancement of packet
radio technology in the 1970s. In 1972 ARPA launched the Packet
Radio Program, aimed at developing techniques for the mobile
battlefield, and SATNet, an experimental satellite network. In 1983
ARPA launched a second-generation packet radio program, Survivable
Adaptive Networks, to demonstrate how packet radio networks could
be scaled up to encompass much larger numbers of nodes and operate
in the harsh environment likely to be encountered on the mobile
battlefield.
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TABLE 1-7 Selected Big Low Earth Orbit
(LEO)/Medium Earth Orbit (MEO) Systems
System
Organization
Number of Satellites
Orbit
Coverage
Data Rate (kbps)a
Year Operational
Globalstar
Loral/QUALCOMM
48
LEO
Global
9.6
1998
Iridium
Motorola
66
LEO
Global
2.4
1998
Odyssey
TRW
12
MEO
Global
9.6
1998
Teledesic
Teledesic
240
LEO
Global
20–2,000
2002
ICO
ICO Global Communications
10
MEO
Global
2.4
1999
Archimedes
European Space Agency
5–6
MEO
Europe, Asia, Canada
256
After 2000
a Kilobits
per second.
SOURCE: Reprinted from Abrishamkar and Siveski
(1996) with permission. Copyright © 1996 by IEEE.
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•
Interactive services (terminal access to host, remote LAN
access, games); and
•
Multicast service (subscription information services, law
enforcement, private bulletin boards).
The first commercial mobile data network was Ardis, a private
network developed in 1983 by IBM Corporation and Motorola to enable
IBM to provide computing facilities in the field. By 1990 Ardis was
deployed in more than 400 metropolitan areas and 10,700 cities and
towns using 1,300 base stations. By 1994 Ardis (since then owned by
Motorola) provided nationwide roaming for approximately 35,000
users, at a rate of 45 million messages per month, and a data rate
of 19.2 kbps.
In 1986, Swedish Telecomm and Ericsson Radio Systems AB
introduced Mobitex and deployed it in Sweden. This system is
available in the United States, Norway, Finland, Great Britain, the
Netherlands, and France. The system supports a data rate of 8 Mbps
and nationwide roaming (international roaming is planned). This
service is distributed by RAM Mobile Data in the United States,
where by 1994 it had 12,000 subscribers. A total of 840 base
stations are connected to 40 switching centers to cover 100
metropolitan areas and 6,300 cities and towns.
Cellular digital packet data (CDPD) technology was developed by
IBM, which together with nine operating companies formed the CDPD
Forum to develop an open standard and multivendor environment for a
packet-switched network using the physical infrastructure and
frequency bands of the AMPS systems. The CDPD specification was
completed in 1993 with key contributions from IBM, McCaw Cellular
Communications, Inc., and Pacific Communications Sciences, Inc.
Deployment of the 19.2-kbps CDPD infrastructure, designed to make
use of idle channels in analog cellular systems, commenced in
1995.
In the 1990s Metricom, Inc., developed a metropolitan-area
network that was deployed first in the San Francisco Bay area and
then in Washington, D.C. The signaling rate of this system is
advertised at 100 kbps but the actual data rate is substantially
slower. The Metricom system uses ''frequency hopping"
spread-spectrum (FHSS) technology in the lower frequencies (around
900 MHz) of the unlicensed industrial, scientific, and medical
(ISM) bands.11
In 1996 the European Telecommunications Standards Institute
(ETSI) standard for mobile data services, trans-European trunked
radio (TETRA), was completed. It is currently being used primarily
for public safety purposes. Work is in progress to enhance the
digital cellular and personal communications technologies. More
recently, the digital cellular standards (GSM, IS-95, PHS, PACS,
and IS-136) have been updated to support packet-switched mobile
data services at a variety of data rates. Key features
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of existing mobile data services are shown in Table 1-8.
Although many services are available, the mobile data market has
grown more slowly than have voice services.
1.7 Wireless Local Area Networks
Wireless LANs provide data rates exceeding 1 Mbps in coverage
areas with dimensions on the order of tens of meters. They are used
for a variety of applications, including the following:
•
LAN extensions in hospitals, factory floors, branch offices, and
offices with wiring difficulties;
•
Cross-building inter-LAN bridges that serve as point-to-point,
high-speed links connecting separate LANs located within a few
miles of each other;
•
Temporary ad hoc networks used in conference registration,
campaign headquarters, and military camps;
•
Temporary wireless access to a wired LAN from a portable device
such as a laptop computer; and
•
Access to centralized computing facilities of a shipboard or
research facility through a wireless device such as a notepad
computer.
In 1990 the Institute of Electrical and Electronics Engineers
(IEEE) formed a committee to develop a standard for wireless LANs
operating at 1 and 2 Mbps. In 1992 the ETSI chartered a committee
to develop a standard for high-performance radio LANs (HIPERLAN)
operating at 20 Mbps.
Table 1-9 indicates the technical features of various LAN
products (including some that use the infrared portion of the
spectrum and are therefore not examined in detail in this report).
The market for wireless LAN products is growing rapidly but not
nearly as fast as the market for wireless voice applications. The
$200 million market for wireless LANs is tiny compared to the
cellular industry, which is worth billions (Wickelgren, 1996).
1.8 Comparison Of International
Research,
Development, And Deployment Strategies
Commercial wireless technologies have followed divergent
evolutionary paths in different parts of the world. For example,
strong contrasts are evident in the transition from
first-generation cellular systems to second-generation systems in
the United States and Europe. At first a single U.S. system was
used for analog cellular communications, AMPS, and every cellular
telephone in the United States and Canada could communicate
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TABLE 1-8 Mobile Data Services
System
Ardis
Mobitex
CDPDa
TETRAb
Metricom
Frequency band (MHz)c
800 bands; 45 kHzd sep.
935–940, 896–961
869–894, 824–849
380–383, 390–393
902–928 (ISMe bands)
Channel bit rate (kbps)f
19.2
8.0
19.2
36
100
RFg
channel spacing (kHz)
25
12.5
30
25
160
Channel access/ multiuser access
FDMAh/ALOHA
Slotted ALOHA
FDMA/ALOHA
ALOHA
FHSSi/BTMAj
aCellular
digital packet data.
bTrans-European trunked radio.
cMegahertz.
dKilohertz.
eIndustrial, scientific, and medical.
fKilobits
per second.
gRadio
frequency.
hFrequency
division multiple access.
iFrequency
hopping spread spectrum.
jBusy tone
multiple access.
SOURCE: Reprinted from Cox (1995) with permission.
Copyright © 1995 by IEEE.
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TABLE 1-9 Wireless Local Area Network
Technologies
Technology
DFIRa
DBIRb
RFc
DSSSd
FHSSe
Channel bit rate (Mbps)f
1–4
10–155
5–10
2–20
1–3
Mobility
Stationary / portable
Stationary with LOSg
Stationary
Stationary / portable
Portable
Range (meters)
15–60
30
10–40
30–200
30–100
Frequency bands
Infrared
Infrared
18 GHzh,
ISMi
ISM
ISM
Systems (companies)
Spectrixlite (Spectrix Corp.); Photolink
(Photonics)
Infralan (InfraLAN); UWIN (Jolt Ltd.)
Altair (Motorola, Inc.); Fast Wave (Southwest
Microwave, Inc.); RediCARDrf (Data Race, Inc.)
Roamabout (Digital Equipment Corp.); ARLAN
(AiroNet Wireless Communications); WaveLAN (Lucent Technologies);
INTERSECT (Persoft, Inc.); AIRLAN (Solectek Corp.); RangeLAN
(Proxim); FreePort (WinData); PRISM (Harris Corp.)
Range-LAN2 (Proxim); PortLAN (RDC Networks);
Netwave (Xircom)
aDiffused
infrared.
bDirected-beam infrared.
cRadio
frequency.
dDirect
sequence spread spectrum.
eFrequency
hopping spread spectrum.
fMegabits
per second.
gLine of
sight.
hGigahertz.
iIndustrial, scientific, and medical.
SOURCES: Reproduced from material in Cox (1995)
and Pahlavan et al. (1995).
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with every base station. By contrast, European users were faced
with a complex mixture of incompatible analog systems. To maintain
mobile telephone service, an international traveler in Europe
needed up to five different telephones. The situation was reversed
by second-generation systems. Now there is a single digital
technology, GSM, deployed throughout Europe (and in more than 100
countries worldwide), whereas the United States has become a
technology battleground for three competitors: GSM (DSC-1900), TDMA
(IS-136), and CDMA (IS-95).
The differences in technology evolution are due in large measure
to different government policies in Europe, the United States, and
Japan, the world's principal sources of wireless technologies.
Three types of government policies influence developments in
wireless systems: policies on radio spectrum regulation, approaches
to R&D, and telecommunications industry structure. The reasons
for the shifts in the above example can be found primarily in
changes in spectrum regulation policies adopted in the 1980s. In
establishing first-generation systems in the United States in the
late 1970s, the FCC regulated four properties of a radio system:
noninterference, quality, efficiency, and interoperability. In the
1980s, deregulation was in vogue and the scope of the FCC's
authority was restricted to noninterference; the other properties
were deemed commercial issues to be settled in the marketplace.
Although this policy stimulated innovation in the U.S.
manufacturing industry, it also meant that operating companies had
to choose among various competing technologies.
In Europe, the main trend in government regulation in the 1980s
was a move from national authority to multinational regulation
under the aegis of the European Community (EC; now the European
Union [EU]). The EC had a strong interest in establishing
continental standards for common products and services, including
electric plugs and telephone dialing conventions. In this context
the notion of a telephone that could be used throughout Europe had
a strong appeal. To advance this notion, the EC offered new
spectrum for cellular service on the condition that the operating
industries of participating countries agree on a single standard.
Attracted by the availability of free spectrum, operating companies
(many of them government-owned) in 15 countries put aside national
rivalries and adopted the GSM standard.
Thus, a new pattern of technical cooperation was established in
Europe. This cooperation was reinforced by the European Commission
(the administrative unit of the EU), which funded cooperative
precompetitive research focusing on advanced communications
systems, first in the Research for Advanced Communications in
Europe (RACE) program and then in the Advanced Communications
Technologies and Services (ACTS) program. In both programs a
consortium of companies and universities
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performs the research. Spectrum management rules continue to
prescribe a single standard for each service, meaning that an
industry consensus is required before a standard is introduced.
Once a technology is established, companies enter the competitive
phase of product development and marketing. This process promotes a
thorough investigation of technologies prior to standardization and
assures economies of scale when commercial service begins. In
preparation for UMTS, scheduled for initial deployment in 2002,
extensive R&D and evaluation of competing prototypes have been
under way since 1994. All of this activity will provide European
industry with a strong technical base for realizing the goals for
mobile communications in the first decade of the next century.
The U.S. approach to communications technology R&D is much
more competitive. Individual companies perform much of this
research in the context of their product marketing plans.
Coordination takes place within diverse standards organizations
such as the Telecommunications Industry Association, IEEE, and
American National Standards Institute. Some interaction also takes
place in the GloMo program, which brings together universities and
industry to fill specific technology gaps identified by DARPA
program managers. But for the most part standards setting is a
competitive rather than cooperative process, with each company or
group of companies striving to protect commercial interests. The
FCC rules for spectrum management allow license holders to transmit
any signals, subject only to constraints on interference with the
signals of other license holders. Similar flexibility is extended
to unlicensed transmissions. As a consequence, there are multiple
competing standards (seven in the case of wideband personal
communications) for wireless service in the United States.
Government policies on industry structure also strongly
influence technology development. After the FCC issued cellular
operating licenses, most of the companies that began offering
cellular service had limited technical resources and relied almost
entirely on vendors and consultants for technical expertise. Even
the cellular subsidiaries of the regional Bell operating companies
had to build a new base of expertise: Under the terms of the
consent decree that broke up AT&T in 1984, these cellular
companies had no access to the abundant technical resources of
Bellcore, the research unit of the regional Bell companies. In this
environment, much of the new wireless communications technology in
the United States has come from the manufacturing industry, with
the result that proprietary rather than open network-interface
standards have proliferated. The published technical standards for
wireless communications were at first confined to the air interface
between terminals and base stations. Eventually the industry
adopted a standard for intersystem operation to facilitate roaming.
Many other interfaces, especially those between switching
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centers and base stations, remain proprietary but the situation
is changing to allow fully open systems.
By contrast, the European cellular operating industry has been
dominated by national telephone monopolies. These companies have
strong research laboratories that participate fully in technology
creation and standards setting. To gain the advantage of
flexibility in equipment procurement, operating companies favor
mandatory open interfaces, a preference reflected in the GSM
standard.
Little has been published concerning the factors that influence
the evolution of wireless communications technology in Japan. In
recent years NTT, the dominant telecommunications operating
company, has provided a strong coordinating mechanism for creating
and standardizing new technology. The biggest success has been PHS,
which entered commercial service in 1995 and attracted 4 million
subscribers in its first year of operation. The initial R&D for
PHS was conducted by NTT, but it licenses many manufacturers to
offer PHS equipment. Now many Japanese companies are cooperating in
a study of wideband CDMA technology for third-generation systems. A
joint experimental trial of one system is scheduled for the end of
1997. In addition to corporate R&D, a government organization,
Research and Development Center for Radio Systems, is a significant
source of wireless communications technology in Japan.
Worldwide efforts to guide the evolution of wireless
communications technology come together in the IMT-2000 project.
National delegations to IMT-2000 reflect their country's policies:
The U.S. delegation pushes for diversity,12the Europeans advocate a structure
favorable to UMTS and its descendants, and the Japanese delegation
favors convergence to a small number of worldwide standards. Other
countries assert their own service needs, which in some cases can
be met by mobile communications satellites and in other cases by
wireless local loops.
1.9 Summary And Report
Organization
The history of wireless communications suggests a number of key
points to be considered in evaluating potential future strategies
for the DOD and DARPA. Wireless technology has now evolved to a
point where the goal of "anytime, anywhere" communications is
within reach. Since 1980 consumer demand for cordless and cellular
telephones has driven rapid growth in wireless services, especially
for voice communications. Wireless data services have not taken off
as yet although expectations are high, given the growth of Internet
applications. Extensive research is under way to develop
third-generation commercial wireless systems, which are expected to
be in place before 2010. These trends suggest that
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the DOD will continue to have an ample selection of advanced
commercial wireless technologies from which to choose.
The DOD, which currently uses a variety of wireless systems
based on 1970s and 1980s technology, is relying increasingly on
commercial wireless products to cope with reductions in defense
budgets and the growing need for flexible systems that can be
deployed rapidly. In the Gulf War, the DOD used commercial
equipment such as GPS receivers and INMARSAT links and found that
performance was comparable to that of technologies designed
explicitly to meet military needs. However, the DOD will continue
to have unique needs for security, interoperability, and other
features that might not be met by commercial products. The gaps
between commercial technologies and military needs are difficult to
identify precisely because, although the DOD has defined its vision
for future untethered systems in general terms, projected
operational needs have apparently not been translated into
technical specifications that conform to the capabilities of
commercial products.
The GloMo program and other military R&D efforts are
attempting to meet DOD's future communications needs and have
produced some useful results. However, none of these programs has
adopted a systems approach to the problem, most notably with
respect to the design of a network architecture. There may be other
unmet needs as well; however, the committee based its work on first
principles rather than an assessment of GloMo. A new strategy may
be needed to identify the needs more specifically as a basis for
determining where to focus DARPA's R&D efforts and where
commercial products will suffice.
The effort to evaluate commercial technologies in light of
defense needs will be complicated by the characteristics of the
U.S. marketplace. In Europe there is a single standard (GSM) for
digital wireless communications, and precompetitive research on new
wireless technologies is carried out in cooperative,
government-funded programs. The U.S. wireless market features a
mixture of competing standards, and most technology R&D is
conducted by individual companies. This environment forces
operators to choose from an assortment of competing
technologies.
The remainder of this report is an attempt to help the DOD
devise strategies for making those choices. Chapter 2 provides
technical background on the many issues that need to be addressed
in designing wireless communications systems, which are extremely
complex. The highly technical discussion may not interest all
readers but is fundamental to any informed analysis of wireless
systems. Chapter 3 explores the opportunities for and barriers to
synergy between the military and commercial sectors in the
development of wireless technologies. Chapter 4 integrates all the
information presented in this report to provide a set of
recommendations for the DOD and DARPA.
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Notes
1. This report does not address unguided
optical communications systems, which use the 103–107 gigahertz frequency band (infrared,
visible, and ultraviolet light), because the commercial products
that operate in these bands are designed for indoor applications
and therefore would not be of great use in military
applications.
2. A protocol is a set of rules, encoded
in software, for performing specific functions.
3. The developments since the mid-1970s,
when the use of computer networks moved beyond the ARPA research
community, paved the way for commercial services. The CSNet
project, funded by the National Science Foundation (NSF) for the
computer science community, eventually led to the NSFNET and a
dramatic increase in the number of interconnected nodes. The
commercialization of Internet service was symbolized by the
decommissioning of the ARPANET in 1990 and privatization of the
NSFNET in 1995.
4. Two types of codes are used to spread
the signal. A long code is reserved for use by the military to
obtain location information within a few meters of accuracy and
timing information within 100 nanoseconds. A shorter code is used
by commercial systems to obtain location information accurate to
within 100 meters.
5. A fourth digital modulation technique,
based on Motorola's iDEN technology, is used by some specialized
U.S. mobile radio services in the lower 800-MHz band to provide
cellular-like voice, trunked radio, paging, and messaging
services.
6. One integrated solution not addressed
in detail in this report is the new generation of public safety
radio networks. These systems are used in both the military and
commercial sectors for applications such as law enforcement and
fire fighting. Until recently these systems were characterized
simply as 25-kilohertz FM voice radios and 9.6-kbps modems. In the
past a municipal law enforcement radio system typically was
deployed as a redundant overlay of towers and repeaters separate
from the radio systems operated by fire, health, highway, and other
municipal departments. Today's tight budgets often force
municipalities to pool departmental funds to upgrade public safety
radios and establish a single system with enough capacity to meet
every user's needs. To assist in this process the Association of
Public Safety Communication Officers (APCO), which includes law
enforcement, highway, forestry, health, and many other municipal
and federal users, recently initiated an ambitious program called
Project-25 to reduce the cost of next-generation radios. APCO
Project-25 seeks to reduce user dependence on proprietary radios
from a single manufacturer (generally the system installer) and
introduce cost competition in the upgrading and replacement market
at the municipality level. The strategy is to standardize a
digital-modulation radio, which would be described as APCO
Project-25 compliant, thus opening up public radio purchasing to a
variety of competing manufacturers. Some radios that are APCO
Project-25 compliant are now available and are being adopted by the
Federal Law Enforcement Radio Users Group (representing radio users
in the Federal Bureau of Investigation, Drug Enforcement Agency,
Secret
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Service, Department of the Treasury, and
other civilian agencies). The APCO Project-25 process has
encouraged an unprecedented level of cooperation among municipal
radio users.
7. These activities are carried out by the
ITU Radiocommunication Sector (ITU-R) Working Party 8/13, later
renamed ITU-R Task Group 8/1.
8. The implementation of standards based
on IMT-2000 in Japan clearly would give Japanese companies early
experience with the technology and perhaps position them to
dominate future world markets for IMT-2000 products.
9. Although optical communications systems
are not addressed in detail in this report, in large part because
the commercial research focuses on indoor applications, the
advantages of laser systems need to be mentioned. A laser produces
optical radiation by stimulating emissions from an electronic or
chemical material. Unlike light produced by incandescent or
fluorescent sources, the resultant beam is coherent and exhibits
extremely low angular divergence, properties that enable
transmissions spanning great distances (i.e., thousands of miles).
The data, voice, images, or other signals are modulated on a beam
of light, which is detected by an optical receiver and decoded. The
transmitter and receiver need to be in direct visual contact, and
so the laser beam is steered in the appropriate direction using
mirrors or other optical elements. Laser communications systems
offer several advantages over RF systems. The main advantage is
high capacity: Systems now under development will support
transmissions in the range of hundreds of megabits per second, with
systems under consideration attaining the gigabits-per-second
range. Another advantage is the low power requirement for
point-to-point communications (orders of magnitude lower than RF
systems). All the energy is focused into a very narrow beam because
the physical dispersion of a laser beam in space is minimal.
Furthermore, laser communications systems offer security benefits
because almost no energy is diffused outside the laser beam, which
is therefore not easily detected by an adversary. This combination
of features makes laser communications systems attractive for
secure transmissions between hub points in mobile, dynamically
changing environments (e.g., between base stations on
vehicle-mounted switching facilities). However, laser systems are
sensitive to interference from other light sources, such as the
sun, and any obstructions of the visual link by dust, rain, or fog.
There is also a risk of damage to the eyes of unprotected
observers. Finally, components for laser-based systems are much
more expensive than those for RF systems and therefore are unlikely
to penetrate the commercial market for some time.
10. These activities are carried out by
the ITU Telecommunications Sector, Study Group 11.
11. The ISM bands (at 902–928 MHz,
2400–2483 MHz, and 5700–5850 MHz) are available for any
wireless device that uses less than 1 watt of transmit power.
12. The United States participates in the
IMT-2000 process in Task Group 8/1 through a delegation led by the
FCC.
Representative terms from entire chapter:
mobile telephone
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